Low-rated circuit breakers



Oct. 10, 1961 A. R. NORDEN 3,004,122

LOW-RATED CIRCUIT BREAKERS Filed Feb. 9, 1959 INVENTOR ALEXANDER R.NORDE/V ATTORNEY Un t d S at Pate fO 3,004,122 I LOW-RATED CIRCUITBREAKERS Alexander R. Norden, New York, N.Y., assignor to FederalPacific Electric Company, a corporation of Delaware Filed Feb. 9, 1959,Ser. No. 792,068 8 Claims. (Cl. 200-416) to trip in response to anycurrent excess over 15 amperes.-

It will thus be appreciated that'such circuit breakers in practicecannot be of critical and delicate construction but must be relativelyrugged. 7

Circuit breakers for this service have heretofore utilized acurrent-responsive bimetal in the overload releasing mechanism, thisbimetal naturally providing a desired time delay characteristic,responding more quickly to a progressively more severe overload andresponding very slowly to a very slight overload. Alternatively, moreelaborate tripping arrangements have been used involving a magnetic coiland a retarded plunger moving Such magnetic in a liquid-filled dash-potarrangement. circuit breakers can be made for virtually any desiredcurrent level, well below the 15 ampere standard commercial minimumrating for which bimetal-type circuit breakers are made; but they arerelatively costly; and the required coil represents a substantial amountof in-' serted resistance which should be minimized, from the point ofview of the protected circuit. While circuit breakers having bimetaloverload release devices have been made under critically controlledconditions for lower values of current than 15 amperes, such designshave generally been quite critical and, accordingly, both' costly anddelicate. I

The foregoing will be appreciated more fully when it is realized thatthe bimetal is commonly a part of an overload releasing arrangement inwhich current-responsive deflection of the bimetal is required toovercome the friction resulting from the relatively heavy latchpressure. The bimetal is heated by the flow of current through thebimetal, functioning as a resistor. Passage of a 15 ampere currentthrough a bimetal has been found to provide a satisfactory deflectionforce for overcoming latch friction in releasing the circuit breaker.

If it should now be considered that a ampere circuit breaker should bebuilt using the same mechanism as is employed in a rugged l5 amperecircuit breaker,

in order to obtain the same physical deflection resulting from a currentin excess of 5 amperes, then the width of the bimetal should logicallybe reduced to of that in a 15 ampere bimetal. The resistance of thebimetal must be increased approximately nine-fold if the same trippingtemperature is to be attained because the current decreases toone-third, and the heating is a func- A bimetal which is tion of thesquare of the current. both thin and narrow enough to meet thiscriterion would not have the necessary mechanical strength to perform asrequired in'typical circuit breakers used in invention is to provide anovel form of circuit breaker to the current passed through the circuitbreaker.

distribution-circuit protection. An object of the present.

3,004,122 Patented Oct. 10, 1961 currents; but it will be readilyunderstood that such re-.

opposite extremity of the bimetal is connected to a flexible braid so asto cause the current in the protected circuit to flow through thebimetal and to provide for thermal deflection. Actually the so-calledfixed; end of the bimetal is carried by the movable contact member inthe illustrative circuit breaker, which contact member is actuallystationary while the circuit breaker is closed, therefore being incondition to sense and respond The bimetal in the illustrative circuitbreaker may be of the same thickness, width and length as that of acircuit breaker rated three times as high (5 amperes as compared to 15amperes, for example) but is divided longitudinally into multipleseries-connected current paths which are quite close to each otherphysically so as to be in good heat-transfer relationship with oneanother. One lengthwise subdivision only is utilized for the mechanicalfunctions of restraining and releasing the circuit breaker controllinglatch.

The additional longitudinal electrically series-connected portions ofthe bimetal serve to raise the temperature of the space about theactiveportion in proportion to the temperature of the active length ofbimetals, in this way sharply. reducing the dissipationof the heat from.

additional mechanically inactive parts of the bimetal also provide bothmechanical and thermal isolation between the conductive braid that isconnected to the bimetal and the mechanically active length of thebimetal. The mechanically inactivelengths of bimetal which are integralwiththe active length serve as a buffer against any mechanicalstressesthat might otherwise be imposed by the braid on the activelength of bimetal, also, loss of heat from the bimetal to thebraid(which would somewhat reduce the temperature rise of the bimetal, andits resulting current sensitivity) is avoided by interposing asubstantial length of self-heating bimetal between the braid and theactive length of the bimetal.

In the circuit breaker disclosed in detail below the present inventionhas special merit. The relatively heavy bimetal (considering thelow-level current effective to' cause tripping) provides both thethermal deflection strength required to effect release of the contactoperating mechanism and further provides the stiffness enabling it toact asa latch in restraining the contact opening mechanism bearingconsiderable endwise pressure in that phase of its operation.Nevertheless the invention has broader application in other trippingarrangements;

as in an arrangement wherein only the deflection strength of the bimetalis needed for shifting and thus releasing a separate latch that normallybears the pressure or where the bimetal is arranged to actin tension inrestraining the contact opening mechanism.

the

V of bimetal useful as a substitute for the bimetal in FIG. 1.

Referring now to FIGS. 1 to 3 of the drawings in detail, there is shownan automatic circuit'breaker 1h provided with a casing constituted bythe insulated body part 14 and a complementary cover of insulation (notshown) that are conventionally secured together in housing definingrelation by rivets through holes 18. The circuit breaker mechanismcomprises a movable contact arm 22 having pivot 21 in the form of ahelical coil spring (viewed endwise) whose ends are captive in pocketsin the confronting walls of the casing. Contact arm 22 carries contact24 that cooperates with stationary contact Ztiunited with and forming aunitary part of solderless terminal 28. At its other end, movablecontact member 22 carries actuator 34} on an insulated pivot. Actuator30 is mechanically connected to handle 32. by a link 34. The movablecontact member 22 is also provided with a current-responsive device 35that restrains actuator 3% against moving clockwise. The overcurrentrelease device 35 as seen in FIGS. 1-3 is in the form of a onepiecebimetal that restrains actuator 30. Overcurrent release device 35includes a bimetal length 36 that is physically and electrically unitedto movable contact member 22. For this purpose, rivet 38 projectsintegrally from contact member 22 and extends through a hole 40 at oneend of the length 36 of the bimetal. At its other end, this length ofbimetal $6 resists pressure of actuator 30 which thus tends to buckle orbow the bimetal portion 36' in the event that a heavy pressure isapplied. When length 36 of the bimetal is heated, it deflects downwardand, overcoming the pressure of actuator 30 and the resulting friction,it releases actuator 36 for opening of the circuit, breaker. Length 36of the bimetal may thus be termed the active length of the bimetal.

The bimetal that here constitutes the release device includes twoadditional mechanically inactive bimetal lengths 42 and 44 electricallyin series with each other and with length 36 as a convolution whichgenerally encircles the active length 36 of the bimetal. The free end ofbimetal length 44 is joined to flexible braid-46, of many soft copperstrands twisted together, and these extend to the plug-in male terminal48 of the circuit breaker.

Portions 36 and 42 of the bimetal are connected to each other by meansof an offset portion 48 which, as best seen in FIGS. 1 and 2, disposesportions 42 and 44 in a plane somewhat below that of bimetal activeportion 36, well clear of contact arm 22.

When handle 32 of the circuit breaker operates the members 34, 30 and 22into position to drive the movable contact 24 against stationary contact26, the portion of the handle between its pivot and its connection toelement 34 acts as one part of a toggle which applies thrust to element34 during the. closing operation. Ultimately, however, when the circuitbreaker is closed, thrust is'applied by element 34 along a line which issomewhat at. an angle (angle A, FIG. 1) to another line extendingbetween the pivot of actuator 30 on member 22 and the pivotal connectionbetween elements 30 and 34. This angle is an important factordetermining the force that actuator 30 applies clockwise against the endportion of the mechanically active length 36 of the bimetal. If this 4angle is made very small, then the circuit breaker action is veryuncertain, bearing in mind the dimensional variations to be expected inmanufacture of such circuit breakers.

In practice the angle is such as to impose a substantial endwisepressure on the release device 35. This in turn means that the bimetalwhen heated by overload current through the circuit breaker is requiredto develop substantial deflection force in consistently and reliablyovercoming the latch friction resulting from the latch pressure and thebimetal must be still so as to avoid bowing andwbuckling as a result ofthat end pressure. These mechanical considerations are satisfied yetresponse to unusually low current levels is achieved in the novel yetsimple manner shown. The left-hand end of the bimetal length 36 iselectrically connected and mechanically fixed to movable contact arm 22(which of course is stationary when the circuit breaker is closed). Therighthand end is thermally deflected to release latched member 30 whichit normally restrains, and that latching end provides the physicalsupport for the other lengths of himetal 42 and 44 which thus need nofurther support or insulation. The right-hand end of length 44 of thebimetal is joined to copper braid 46. When the circuit breaker isoperated for opening and closing the contacts, the resilient bimetallengths 42 and 44 minimize trans mission of stress from braid 46 (whichhas one end stationary at terminal 48) to the critical latch end ofactive bimetal length 36. Mechanical disturbance imposed at this point,especially where active'length 36 of the bimetal is slender forlow-circuit response, could seriously vary the overload tripping levelof the circuit breaker.

' Bimetal lengths 42 and 44 manifestly do not contribute to the.deflection force in an overload release operation. Nevertheless theypromote high sensitivity or response to low current levels by raisingthe temperature of the space about the active length of'bimetal so as toinhibit heat dissipation from that active length of bimetal. This meansthat the temperature and consequent deflection resulting from. a givencurrent will be much higher with the inactive bimetal lengths than itwould be if they were omitted. In an example, a circuit breaker havingonly an active length of bimetal tripped at about 10 am peres whereas alike circuit breaker with a bimetal having the shape in the. drawings,including an active length of equal cross-section and of this samebimetal stock' tripped at 4 amperes, latch element 36 extending equaldistances across the ends of the bimetals when cold in those circuitbreakers.

In FIGS. 1-3 the active length of bimetal is sheared from the laterallengths, so that. insulating space is realized by oft-setting lengths 42and 44 from active length 36. ,By cutting a. narrow slot in the bimetalstock as in FIG. 3a the lengths of bimetal are provided with in-'sulating separation'and the off-set 48 of FIG. 2 may be avoided; buteven in that instance an off-set may be desir' 1. A circuit breakerincluding a pair of contacts, a

movable. contact member carrying one of said contacts,

trip-free manually operable mechanism for opening andv closing saidcontacts and an overload release means for automatic opening of the.contacts in response to'over current conditions, said overload releasemeans including a latched member, a length of bimetal arranged forlatching said latched member, and at least one additional length ofbimetal integral with said first mentioned length of bimetal andsupported. only by said latching length. of;

bimetal, said additional length of bimetal being closely adjacentthereto and electrically in series therewith, said latching length ofbimetal being united at one end thereof to said movable contact memberand said lengths of bimetal being thereby mounted for bodily movementwith said movable contact member, a connecting conductor of braid havinga joint to said additional length of bimetal, said connecting conductorhaving a stationary portion spaced from said joint, said additionallength of bimetal providing environmental heating to resist dissipationof heat developed in said first mentioned length of bimetal and saidadditional lengths of bimetal further providing mechanical isolationbetween said conductor and said latching length of bimetal.

2. A circuit breaker in accordance with claim 1 wherein said bimetalincludes plural lengths of bimetal in series with and extending alongsaid latching length of bimetal but on opposite sides thereof.

3. A circuit breaker including a pair of contacts, a movable contactmember carrying one of said contacts, trip-free manually operablemechanism for opening and closing said contacts and an overload releasemeans for automatic opening of the contacts in response to overcurrentconditions, said overload release means including a latched member, afirst length of bimetal arranged to deflect in response to self-heatingand thereby to trip said latched member, and at least one additionallength of bimetal integral with said first mentioned length of bimetaland supported only by said first length of bimetal, said additionallength of bimetal being closely adjacent thereto and electrically inseries therewith, said first length of bimetal having one end thereofunited to said moving contact member and said lengths of bimetal beingthus mounted for bodily movement with said movable contact member, aconnecting conductor of braid having a joint to said additional lengthof bimetal, said connecting conductor having a stationary portion spacedfrom said joint, said additional length of bimetal providingenvironmental heating to resist dissipation of heat developed in saidfirst mentioned length of bimetal and said additional lengths of bimetalfurther providing mechanical isolation between said conductor and saidfirst length of bimetal.

4. A circuit breaker including a pair of separable contacts, manuallyoperable mechanism for opening and closing said contacts, and overloadrelease means, said means including a bimetal arranged to causeautomatic release of the circuit breaker, said bimetal having integralserially connected lengths of bimetal forming part of a current paththrough the circuit breaker, said lengths of bimetal being in closeproximity to each other, at least one of said lengths being mechanicallyactive in operating the overload release means in dependence on thecurrent through the circuit breaker and at least one of said bimetallengths being inactive mechanically in relation to the release mechanismbut being effective due to self-heating to heat the space adjacent theactive length of bimetal and thereby to inhibit heat dissipationtherefrom.

5. A circuit breaker including a pair of separable contacts, manuallyoperable mechanism for opening and closing said contacts, and anoverload release means, said means including a bimetal arranged to causeautomatic release of the circuit breaker, said bimetal having integralserially connected lengths of bimetal forming part of a currentpaththrough the circuit breaker, said lengths of bimetal being in closeproximity to each other, one of said lengths being mechanically fixed atone end thereof and being mechanically active at the other end thereoffor releasing the overload release means in dependence on the currentthrough the circuit breaker and another of said bimetal lengths beingdisposed adjacent to said one length and supported substantiallyexclusively by said active end 01" said one bimetal for effectingenvironmental heating of the space about said one length and thereby tominimize heat dissipation therefrom and to provide mechanical ism lationbetween said active end of said one length of bimetal and the end ofsaid other length of bimetal remote means including a release elementand a bimetal cooperable therewith for effecting automatic release ofthe circuit breaker, said bimetal having integral serially connectedlengths of bimetal forming part of the current path through the circuitbreaker, said lengths of bimetal being in close proximity to each other,one of said lengths having one end thereof fixed'and the opposite endthereof cooperable with said release element and thereby being mechanically active for releasing said overload release means independence on the current through the circuit breaker and another ofsaid bimetal lengths having one end thereof supported at saidmechanically active end of said one length and a braid conductorconnected to the other end of said other length of bimetal providing anelectrical connection mechanically remote from said active end of saidone length of bimetal.

7. A circuit breaker including a pair of separable contacts, manuallyoperable mechanism for opening and closing said contacts, and anoverload release means, said means including a release element and abimetal cooperable therewith for effecting automatic release of thecircuit breaker, said bimetal having three integral lengths seriallyconnected to form part of the current path through the circuit breaker,two of said lengths being disposed at opposite sides of the third lengthand being mechanically inactive, said third length having one end fixedand its opposite end connected to one of said two lengths and saidopposite end being mechanically active in efiecting operation of saidoverload release mechanism in dependence on the current through thecircuit breaker, the second of said two lengths of bimetal having anelectrical terminal connection physically adjacent to said active end ofsaid third length.

8. An overload release device comprising a latched element, a firstlength of bimetal fixed at one end and having its opposite end arrangedas a latch to resist endwise pressure of said latched element, a secondlength of bimetal extending integrally from said first length at saidopposite end thereof and wholly supported thereby, and said secondlength extending laterally along one side of said first length ofbimetal, a third length of bimetal extending integrally from and whollysupported by said. second length at the end thereof remote from saidlatch end of said first bimetal, said third length extending along theside of said first length opposite the second length of bimetal, saidsecond and third length of bimetal being mechanically inactive, theremaining end of said third length of bimetal and said fixed end of saidfirst length of bimetal having electrical connections thereto forself-heating control cur- Switzerland July 1, 1935

